mkosi: Mark minimal images as Incremental=relaxed

mkosi: Mark minimal images as Incremental=relaxed

Building these over and over again is extremely wasteful. Let's just
build them once and not touch them again unless -ff is specified or
their cache is out-of-date, which is exactly what Incremental=relaxed
does.

This means mkosi.extra/ and such changes won't be picked up for these
anymore unless -ff is used, but that slight annoyance is totally
outweighed by the time saved building the images.